Polycyclic aromatic hydrocarbons (PAHs) are a group of aromatic compounds containing two or more fused benzenoid rings in linear, angular, or cluster structure. They are ubiquitous compounds that are formed naturally during thermal geologic reactions, plant fossilization, and bacterial reactions, or formed anthropogenically during mineral production, combustion of fossil fuels in heat and power generation, refuse burning, coke oven, pyrolysis, and forest and agricultural fires. The major sources of PAHs are crude oil, coal, and oil shale. Hydrophobic, recalcitrant, and bio-accumulating, PAHs adsorb strongly to suspended particulates and biota, and accumulate in soil and sediment, resulting in serious soil contamination problems. Health concerns of PAHs arise from their toxicity, mutagenicity, and carcinogenicity. Of the 16 PAHs listed by the US EPA as priority pollutants due to their toxic and mutagenic nature, six are also known to be carcinogens. PAHs are known as active carcinogens. Their presence is an indicator of industrial pollution, and they are widely distributed in contaminated environments, particularly prevalent in burnt organic matter, air, and contaminated soil.
Both biological and chemical techniques have been used for the remediation of PAHs, although bioremediation is generally found to derive cost and technical advantages. While low-molecular-weight PAHs are susceptible to biodegradation, high-molecular-weight (HMW) PAHs that are highly mutagenic and carcinogenic remain recalcitrant. The refractory nature of HMW PAHs is partly attributed to their low aqueous solubility and bioavailability, with their degradation rates possibly limited by dissolution or desorption.
Chemical oxidation using electrophile O3 has been seen as a treatment for PAH compounds in the aqueous phase or in solution. PAH compounds, such as benzo[a]pyrene, in organic solvents or in various aqueous solvents have been treated with ozone to form oxygenated products. However, these are limited in their utility for remediation because they required that the PAH compounds be in solution. Since the solubility of many PAH compounds is water is low, such solution treatment with ozone is limited as it treats only the more soluble compounds.
PAH compounds may be more soluble is certain organic solvents such as ethylene or methylene chloride, but these solvents in themselves present environmental problems and are accordingly undesirable for environmental remediation applications. In addition, the reaction products of the ozone and PAHs are often insoluble in these organic solvents, causing insoluble solid precipitates. Addition of water to these systems to solubilize the intermediates creates a multiphase system that is difficult to handle.
One of the more severe environmental problems involving PAHs is derived from oil spills. Oil spills are known for causing long-term and severe damage to environment. Biodegradation, volatilization, oxidation, and photochemical reactions alter a limited amount of the oil; the remainder of the oil is dissolved into water, and/or dispersed into soils. Many high molecular weight and hydrophobic compounds such as polycyclic aromatic hydrocarbons (PAHs) and aromatic sulfur compounds are accumulated due to their toxicity and poor water solubility, thus inaccessible to microbes and even to chemical oxidant such as O3 in the aqueous phase.
Petroleum released into environments have been remediated with a wide range of chemical, physical, and biological processes. Different fractions of oil spills can transform or degrade through evaporation, plant uptake, and dissolution into water, adsorption by soil matrix, photo-oxidation, and biodegradation. Among all the attenuation phenomena, biodegradation is the primary mechanism for contaminant destruction. Biodegradation of oil in terrestrial and aquatic environments is currently the most widely accepted option for petroleum-contaminated sites. The biological degradation of oil can be taken place in aerobic or anaerobic environments, although aerobic biological oxidation is regarded as more efficient. In low oxygen conditions, such as in a oil-polluted ground water environment, the biotransformation of hydrocarbons can also occur when the nitrate, sulfate, carbon dioxide, and ferric iron were utilized as alternate electron acceptors. However, petroleum degradation under anaerobic condition is generally considered to be difficult due to the limited growth substrate, electron acceptors, and enzymatic activities. Accordingly, aerobic biological oxidation of hydrocarbons is considered to be the major biodegradation processes.
The preferential biodegradability of fractions in the crude oil spill has been reported as the n-alkanes>branch alkanes>cyclic alkanes>aromatics. In addition, volatile aromatic fractions (i.e. benzene and toluene) of oil have short residence times in the environment. Having a low preference for biodegradability with low bioavailability, low enzymatic activity, and a low volatility, high-molecular weight and hydrophobic compounds of petroleum accumulate. For example, the cyclic-alkanes and polycyclic aromatic hydrocarbons from oil spills will stay in the environment for a long period of time. Especially, PAHs are relatively stable and diagnostic constituents of petroleum. The biodegradability of polycyclic aromatic compounds are limited by their toxicity and water solubility because of most of the biodegradation occurring in the water or water-oil interface. Thus, the accumulation, persistence, and mobility/leaching potential of toxic PAHs even with effective bioremediation are still the major health and environmental concerns. In other words, although bioremediation can be a cost-effective method to remove considerable amounts of oil spills, the contaminant concentration cannot be completely eliminated because of these persistent PAH compounds.
Ozone for the oxidations of PAH compounds in aqueous solutions has been found to effective for those compounds in solution, but is not effective for these compounds that are essentially insoluble. Wastewater containing recalcitrant organic compounds has been successfully treated with ozone. Many studies on degradation of PAHs by ozone proven can improve the solubility and decrease the toxicity of PAH compounds.
In summary, the prior-art shows (1) treatment of water soluble PAH compounds with ozone in water, generally for waste water treatment, (2) treatment of PAH in non-polar solvents with ozone, sometimes in conjunction with a non-miscible solvent to form two phases. The main problem with these systems is that non-soluble PAH compounds are not available to water solution and escape reaction with ozone. The problem with non-polar solvents is that such solvents are often toxic in themselves, and introduce their own environmental problems. In addition, the non-polar solvents do not effectively dissolve the oxygenated reaction products of ozone and the PAH compounds. Thus, these compounds can precipitate from the solvent and are not removed.